1. Field of the Invention
The present invention relates to a dielectric resonator, a dielectric filter, and a dielectric duplexer of the type comprising a dielectric block and electrodes formed of conductive films which are formed on internal and/or external surfaces of the dielectric block, and to a communication device using the same.
2. Description of the Related Art
In general, a dielectric resonator for use mainly in the microwave band includes a prism-shaped or cylindrical dielectric block having a through-hole coaxially formed therein, an inner conductor formed on an inner surface of the through-hole, and an outer conductor formed on an outer surface of the dielectric block, so as to operate as a dielectric coaxial resonator. Another form of dielectric resonator includes a rectangular dielectric block having a plurality of through-holes formed therein, an inner conductor formed on an inner surface of each of the through-holes, and an outer conductor formed on an outer surface of the dielectric block, such that a single dielectric block incorporates a plurality of dielectric resonators, so as to operate as a filter or duplexer having multiple stages of resonators.
Such a coaxial resonator, or a filter using the coaxial resonator, is compact as a whole, and is characterized in that the resonator has a high unloaded Q-factor (Qo).
The Qo of the coaxial resonator greatly depends upon the profiles of the inner conductor and the outer conductor, and it is important to form conductor films each having a precise and smooth surface in order to enhance the Qo. Since the coaxial resonator is structured with a conductor film deposited on an inner surface of a hole formed in the dielectric block, it is difficult to improve the characteristics of the inner conductor film, as compared with those of the outer conductor formed on an outer surface of the dielectric block.
For example, in the case of a transmission filter or a duplexer used as a shared antenna device in a circuit which encounters relatively large power, as electronic components to be incorporated in the circuit are made more compact, with reduced power consumption, the demand increases for further reduction of losses caused by the resonator or the filter.
The resonator losses typically include conductor loss caused by the conductor films, dielectric loss in dielectric portions, and radiation loss due to radiation to the outside. Of these losses, the conductor loss accounts for the greatest part of the resonator losses, and therefore, it is of significance how to reduce the conductor loss.
An effective approach to reducing the conductor loss is to use a conductor material having a high electric conductivity and to increase the film thickness. In high-frequency-band devices such as microwave band devices, however, current flow is concentrated only in the skin-depth portion of the conductor film. Hence, making the conductor film thicker than the skin depth provides substantially no advantage with respect to reducing the conductor loss.
An extremely effective approach is described in Japanese Patent Application No. 11-3 14658 (which corresponds to U.S. Pat. Ser. No. 09/707,264 filed Nov., 6, 2000), in which a conductor film has a thin-film, multi-layer electrode structure, in which thin-film conductor layers and thin-film dielectric layers are alternately laminated.
Another extremely effective approach is described in Japanese Patent Application No. 11-375 194 (which corresponds to U.S. Pat. Ser. No. 09/750,947 filed Dec. 28, 2000), in which an inner conductor of a coaxial resonator is formed of plurality of multiplexed helical lines.
However, it is difficult to form either a thin-film, multi-layer electrode or to form a multiplexed inner conductor on an inner surface of a hole having a small inner diameter.